A photodiode array is a device in which semiconductor devices each formed by connecting a thin film transistor (hereinafter, referred to as a TFT) and a photodiode are aligned in a matrix pattern. Oxide semiconductor is used as an alternative material for amorphous silicon semiconductor that is used for TFTs included in a photodiode array.
In other words, a photodiode array is one type of image sensor, and an element thereof is formed by a photodiode formed using hydrogenated amorphous silicon and a TFT. In the mechanism thereof, photoelectric conversion of light is performed using the photodiode, and acquired electric charge is read from a signal line through the TFT. Particularly, the TFT is required to have high off-resistance for preventing leakage of generated electric charge and low on-resistance for easily moving electric charge to the signal line at the time of reading the electric charge. Thus, recently, a photodiode array in which oxide semiconductor as a material having a high on/off ratio is mounted receives a lot of attention.
The oxide semiconductor is one type of semiconductor configured by a metal cation and an oxide ion. In the oxide semiconductor, oxide ions form a valence band, and a metal cation forms a conduction band. Thus, here, in a case where the oxide ions come out, remaining electrons present in a remaining site after the oxide ions come out move from the valence band to the conduction band, thereby electricity flows. At this time, the remaining site after the coming-out of the oxide ions is called oxygen vacancy. In other words, the oxygen vacancy serves as a donor, and, as the oxygen vacancy increases, the density of electrons flowing through the conduction band increases, whereby sheet resistance of the oxide semiconductor decreases. In addition, it is known that the conductivity increases in a case where hydrogen is introduced into oxide semiconductor (see U.S. Pat. No. 5,569,780). Furthermore (see Nomura, K. et al. Applied Physics Letters, 93 (2008) 192107), it is represented that, in a case where annealing is performed in oxygen atmosphere containing water vapor, as the humidity is higher, the mobility is improved more, and a threshold shift and an S value decrease. Here, the S value is also called sub-threshold swing value and represents a switching characteristic of a transfer characteristic. The smaller the S value is, the better the rising is. In addition, in Nomura, K. et al. Applied Physics Letters, 93 (2008) 192107, it has been disclosed that, in a case where an anneal gas is changed from a nitrogen gas to an oxygen gas, an increase in the density of oxygen vacancy is suppressed, and an excessive rise of the electric conductivity can be suppressed. Furthermore, it is known that, in a case where hydrogen penetrates into an oxide semiconductor layer, the sheet resistance of the oxide semiconductor decreases (see Gosain, D. P. et al. Japanese Journal of Applied Physics, 48 (2009) 03B018). As disclosed in such prior art documents, by introducing specific gas molecules, the electric characteristics of the oxide semiconductor diversely change.
However, there is a problem in the oxygen vacancy that the density easily changes also after film formation. A change in the density of oxygen vacancy influences the property of the oxide semiconductor. For example, also in a case where specific gas processing is not performed, in the oxide semiconductor, hydrogen and oxygen easily go in or out according to the heat, and, in a case where a membrane stress is received, oxygen vacancy generation energy increases or decreases according to the type of the stress such as a compressive stress or a tensile stress, and a threshold is shifted. In this way, the oxide semiconductor is quite sensitive to a component contained in the film or a surrounding environment (see Liu, S.-E. et al. IEEE Electron Device Letters, Vol 32, No 2, February 2011, 161). In a case where IGZO is used in the oxide semiconductor, relating to the diffusion or penetration of hydrogen or oxygen to the inside or outside of the film that is a problem of the former, it has been disclosed that (see JP 2012-49209 A), by increasing the ratio of gallium having high binding energy for oxygen, the discharge of oxygen to the outside of the film can be suppressed. However, a solution for the membrane stress that is a problem of the latter has not been reported until now. For this reason, in order to decrease the membrane stress, the material of the protective film is limited to a material having a low membrane stress, and the number of stacked layers and the film thickness are restricted, whereby a burden on the design increases.
In addition, by combining an oxide semiconductor TFT and a photodiode, another problem occurs. The problem is that, when a film of amorphous silicon is formed thick, a large amount of hydrogen plasma is generated. FIG. 1 is a cross-sectional view of the element structure of a conventional photodiode array. Here, as illustrated in FIG. 1, in a case where an oxide semiconductor TFT is built in a photodiode array, a hydrogen concentration or an oxygen vacancy density of the inside of the oxide semiconductor film changes at the time of forming an amorphous silicon film of an upper layer changes, and the semiconductor characteristic is changed. In other words, for example, as illustrated in the prior art document 1, the hydrogen concentration of the inside of the oxide semiconductor layer increases, and there are cases where the off-characteristic of the TFT is degraded. FIG. 2 is a graph that illustrates the transfer characteristic of a TFT using oxide semiconductor according to a conventional technology 1. In FIG. 2, the vertical axis represents the drain current, and the unit thereof is ampere (A). In FIG. 2, the horizontal axis is the gate voltage, and the unit thereof is volt (V). As illustrated in FIG. 2, in a test described in the prior art document 1, in a case where hydrogen plasma is emitted in the state of an initial characteristic A having high hysteresis, as denoted by “B”, the off-characteristic is not represented. Meanwhile, when a photodiode is formed as a film after the formation of the oxide semiconductor as a film, a large amount of hydrogen generated at the time of chemical vapor deposition (hereinafter, referred to as CVD) film formation is accumulated in a protective film disposed between an oxide semiconductor layer and a hydrogenated amorphous silicon layer, and a part thereof arrives up to the oxide semiconductor layer. For this reason, hydrogen arriving up to the oxide semiconductor layer degrades the initial characteristic, hydrogen accumulated in the protective film gradually diffuses according to the heat and finally penetrates into the oxide semiconductor layer, thereby degrading the reliability. Accordingly, the semiconductor characteristic may be easily caused to be in the situation as denoted by “B” illustrated in FIG. 2.
A first countermeasure for such a problem is based on a characteristic recovery property of the oxide semiconductor. For example, in the prior art document 1, it has been checked that, by performing a steam treatment of oxide semiconductor that is in the state B, as denoted by a characteristic C, the off-characteristic is recovered. Thus, there is a possibility that the characteristic of oxide semiconductor, which has been degraded once, is recovered by performing appropriate post-processing such as gas annealing. However, in the conventional technology, there is no case where post-processing for enhancing the characteristic of an oxide semiconductor layer is performed after forming protective films stacked thick like first to third protective films 7 to 14 illustrated in FIG. 1. The reason for this is that, even in a case where gas annealing is performed from above thick protective films, a quite long diffusion time is required for a gas to arrive at and penetrate into an oxide semiconductor layer that is a target for recovering the characteristic, and accordingly, the efficiency is low. In addition, it is difficult for gas molecules, particularly, having a large molecular size to permeate the oxide semiconductor layer, and accordingly, an effect of the gas annealing cannot be acquired. In a structure, in which a photodiode is arranged on an upper layer of the oxide semiconductor layer, assumed in the present application, in addition to a protective film for the oxide semiconductor, a protective film for the photodiode is required as well. Thus, many layers are present as upper layers of the oxide semiconductor layer, whereby a thick film is formed. Therefore, according to the conventional technology, it is difficult to solve the problem described above.
A second countermeasure plan is to arrange a protective film that prevents hydrogen from penetrating into oxide semiconductor. However, hydrogen plasma and a hydrogen molecule have extremely small particle sizes and can easily pass through a general silicon oxide film (hereinafter, also referred to as a SiOx film) or a silicon nitride film (hereinafter, also referred to as a SiNx film) and cannot be blocked. Thus, in a case where the oxide semiconductor is formed as a film before hydrogenated amorphous silicon, it is difficult to prevent the penetration of hydrogen.
A third countermeasure plan is a method in which a photodiode is formed as a film first, and oxide semiconductor is formed as a film later.
However, such a method has problems 1, 2, and the like described below.
1. In order to acquire a good oxide semiconductor characteristic, high-temperature annealing is required, and in a thick photodiode layer, the high-temperature annealing causes peeling.
2. Since the photodiode layer is arranged in a lower layer, the number of stacked films of the upper layer increases, and the amount of light arriving at the photodiode decreases, thereby causing the degradation of quantum efficiency.
Here, the quantum efficiency is a photoelectric conversion ratio, and, as this value is larger, the photosensitivity is superior.
The problems can be summarized as below.
Problem of Membrane Stress
1-1. The threshold of the TFT is not stabilized due to the influence of a membrane stress.
1-2. A film having a high membrane stress cannot be formed in an upper layer of the oxide semiconductor layer.
Problem Occurring Due to Combination with Photodiode
2-1. The off-characteristic is degraded due to hydrogen generated at the time of forming photodiode films.
2-2. In a case where a photodiode is arranged in a lower layer, other problems occur.
An object of the present invention is to provide a semiconductor device enabling the recovery of initial characteristics and reliability of oxide semiconductor, which is degraded due to the influence of hydrogen generated after formation of an oxide semiconductor layer, without arranging a photodiode in a lower layer.